Electric cable and method of manufacture



April 3. 1963 L. JACHIMOWICZ 3,087,007

ELECTRIC CABLE AND METHOD OF MANUFACTURE Filed Feb. 4, 1960 INVENTOR. LJACHIMO W/CZ BY United States Patent 3,987,007 ELECTRIC CABLE AND METHQD @F MANUFACTURE Ludwiir Jaehimowiez, Elizabeth, N.J., assignor to General Cable Corporation, New York, N.Y., a corporation of New Jersey Filed Feb. 4, 1960, Ser. No. 6,729 7 Claims. (Cl. 174-110) This invention relates to electric cable and, more particularly, to an improved electric cable having a longitudinally folded metallic tape around the cable core and to an improved method of making such cable.

The art is familiar with cables having a flat or corrugated metallic tapelongitud-inally folded around the cable core over which is extruded a jacket of thermoplastic insulating material. Such cables are well known in telephone cable construction under the trade name of Alpeth cables.

When the metallic tape is not soldered or bound at the overlap, it tends to spring open after leaving the forming tool. The outer edge of the cylindrically folded tape thus raises and cuts into the thermoplastic sheath extruded thereover. This lifting has been recognized by the art and is observable by the visible, and often pronounced, axial indentation formed in the plastic jacket by the edge of the tape. The indentation or step may be in the range of -25 mils deep. Often the edge of the metallic tape will lift enough to cut through the entire wall thickness of the jacket.

To compensate for the undesirable reduction of the wall thickness of jacket over the outer edge of the tape, the art has customarily applied heavier jackets over cables using unbound metallic tapes than over cables in which the metallic tape is bound at the edges. In addition to the fact that the heavier wall increases the manufacturing cost of this type of cable, the application of the heavier wall does not solve the basic problem.

The art has also attempted to solve this problem by using additional dies placed inside of the tuber head. Such attempts have been unsuccessful in practice.

The art has also attempted to provide means for applying a helical wrapping over the longitudinally folded metallic tape. Such attempts have been unsuccessful commercially due to the difficult problems of synchronization of the wrapping element with the already complicated folding and extrusion elements of fabricating machines.

It is therefore one object of this invention to provide an improved cable construction obviating the disadvantages of the prior art.

It is a still further object of this invention to provide an improved cable having a longitudinally folded metallic tape on the core thereof over which is extruded a jacket of uniform thickness.

It is a further object of this invention to provide a cable construction and a method for fabricating such cable in which the longitudinally folded tape is restrained against springing open during extrusion of the overlying jacket.

It is a still further object of this invention to provide an improved method for a cable fabrication in which a longitudinally folded tape is prevented from springing open by application of an axially extended tape bridging the overlap during application of the jacket extruded thereon.

It is a still further object of this invention to provide an improved method of cable fabrication in which a longitudinally folded tape is prevented from springing open by application of an axially extending tape bridging the overlap thereof and by circumferential pressure on the jacket extruded thereover.

3,087,967 Patented Apr. 23, 1963 2 ice In accordance with these objects there is provided in a preferred embodiment of this invention a cable construction comprising a cable core and a metallic tape longitudinally folded thereover. An axially extending tape is applied to bridge the overlap of the folded metallic tape and to prevent raising of the edge of the tape during extrusion of the plastic jacket thereover. The jacket is of uniform wall thickness dimensioned to provide the requisite insulation and strength in the finished cable.

In accordance with the method of this invention the cable core is assembled and a metallic tape longitudinally folded thereover. A tape is applied to bridge the axially extending overlap, and the jacket extruded directly thereover. The edge of the metallic tape is prevented from rising by concentrically applied pressure on or by the jacket during setting of the extruded packet. Thus the integrity of the jacket is maintained and the wall thickness is uniform.

This invention may be more easily understood by reference to the following description taken in conjunction with the accompanying drawings of which:

FIGURE 1 is a perspective view of the cable in accordance with the prior art as it emerges from the metallic tape folding die.

FIGURE 2 is an enlarged cross sectioned view of the cable construction known in the art.

FIGURE 3 is a sectioned view of a cable in accordance with this invention within the tuber head, illustrating the method of cable construction.

FIGURE 4 is an enlarged cross sectional view of the cable shown in FIGURE 3, and

FIGURE 5 is an enlarged, fragmentary, cross sectioned view of the cable shown in FIGURE 4.

In FIGURE 1 there is shown a cable such as a telephone cable being formed in accordance with the practices of the prior art. The cable has metallic tape 10 longitudinally folded over the cable core 11 by the die 12. In the Alpeth cables, which are typical of this construction, the tape comprises an aluminum tape.

After folding of the metal-lic tape longitudinally about the cable core, the cable is passed through an extrusion head and plastic is extruded thereover in an enca sing jacket shown best in FIGURE 2.

In FIGURE 2; there is shown a fabricated cable comprising the cable core 11, a longitudinally folded metallic tape 10 thereover, and an enclosing jacket 13 of insulating material, such as polyethylene.

After the longitudinally folded tapes leave the forming die, the springiness thereof will tend to separate the edges which are overlapped by the dies. The edge springs up cutting into the soft jacket as illustrated at 14 to weaken the jacket and in some cases completely cut through a jacket of normal insulating wall thickness. The use of a vacuum extrusion press, though having the tendency to hold the tape 10 in close contact with the conductor core, exaggerates the deformation at the overlap since the vacuum will tend to draw the soft, but jacketing material between the overlapped edges. Since the outer edge of the tape is the electrical weak point of the cable, the reduction of the jacket wall at this point seriously degrades the performance capabilities of the resultant cable.

For this reason the art has resorted to extrusion of a much heavier jacket with this type of cable construction than that used with other cable types.

The method of fabricating cable in accordance with this invention may best be understood by reference to FIGURE 3.

In FIGURE 3 there is shown a cable comprising a cable core 11 around which a longitudinally folded metallic tape 10 is formed to encase the core. As the tapered cable is fed through the tuber head schematically shown as 16, a tape 15 is fed from the storage roll and applied axially to the cable to bridge the overlap of the folded metallic tape. A tape found suitable is a polyester tape 1-3 mils thick and 14% inches wide. It will be noted, however, that other tapes may be employed since the primary function of the tape is to serve as a mechanical bridge.

As the cable with the tape applied passes through the tuber head, a concentric ring of plastic insulating material 13 is extruded thereover at a rate related to the rate of withdrawal of the cable to form an annular jacket of the desired wall thickness for the application intended. The jacket may be formed of conventional jacketing material having thermoplastic characteristics and of wall thickness conventional in such jacketing. The extrusion head may be evacuated in conventional fashion so that atmospheric pressure will press the jacket 13 against the outer surface of the formed cable. Such extruders are known to the art in the so-called vacuum-sleeving process. Evacuation of the extruder head results in atmospheric pressure, applied concentrically about the extruded jacket, forcing the tape down against the overlap of the folded metallic tape as is best shown in FIGURES 4 and 5 to which reference is made.

As will be noted from FIGURES 4 and 5, the tape 15 forms a smooth bridge over the edges of the metallic tape at the overlapping edges thereof. The extruded jacket is deposited over the smooth surface of the bridge and the concentric pressure of the jacket will cause the overlapping edges of the metallic tape to slide together tightening the tape about the core without causing the outer edge of the metallic jacket to cut into the jacket. It is further noted that the tape 15 allows eflicient use of the vacuum extrusion process since the tape will prevent drawing of the soft, hot plastic into and between the overlap of the longitudinally folded tape.

Similarly, a concentric pressure is applied by shrinking of the jacket when the vacuuni-sleeving process is not used and in place thereof is employed a non-vacuum extrusion followed by a water-cooling bath.

In either event, the jacket wall thickness is maintained uniform. The slight deviation from a truly cylindrical shape as indicated by dotted line 16 does not adversely affect the mechanical and electrical characteristics of the jacket.

The resistance of the jacket to voltage failures, such as that which might be imposed by a lightning stroke, is increased over the construction known to the art. Electrically, the sharp outer edge of the folded tape is the weakest spot around the circumference of the jacket, and by use of this invention, it is ensured that the wall thickness of the jacket is not reduced at this weak spot. Further,

when the tape comprises a polyester film, reinforcement 5 of the dielectric strength at the weak spot is provided,

The cable formed in accordance with the method of this invention is mechanically stronger since the tape, particularly a tough polyester tape, is positioned between the jacket and the sharp exposed edge of the metallic edge.

It has been found that cables may be formed in this manner which have the requisite mechanical and electrical properties for the application intended and in which the jacket thickness is reduced over that of cables known to the prior art, resulting in a decrease in raw material costs of the finished cable.

This invention may be variously modified and embodied within the scope of the subjoined claims.

What is claimed is:

l. The method of jacketing a cable having a longitudinally folded metallic tape encasing the core thereof with overlapping edges which comprises applying a bridging tape axially over the overlapping edges of the metallic tape, extruding a jacket of elastomeric material thereover, and applying a concentric pressure to the jacket to form said jacket about said cable in close contact therewith and to cause the overlapping edges of the metallic tape to slide together and tighten the metallic tape about the core.

2. The method of forming a cable in accordance with claim 1 in which a concentric pressure is applied by evacuating the extrusion mold.

3. The method of forming a cable in accordance with claim 1 in which said tape comprises a tape of high dielectric strength.

4. The method of forming a cable in accordance with claim 1 in which said bridging tape comprises a polyester tape.

5. A cable comprising a cable core, a longitudinally folded metallic tape enclosing said core with the longitudinal edges of the metallic tape in overlapping relation, a bridging tape axially extending along said metallic tape and covering the overlapping edges thereof without being secured thereto, and a jacket of elastomeric material extruded over said metallic tape and said bridging tape and concentrically enclosing said cable.

6. A cable in accordance with claim 5 in which said bridging tape comprises a tape of high dielectric strength.

7. A cable in accordance with claim 5 in which the jacket has a constant wall thickness across the edges of said overlapped metallic tape.

References Cited in the file of this patent UNITED STATES PATENTS 2,795,264 Pechy June 11, 1957 2,939,905 Canfield June 7, 1960 FOREIGN PATENTS 850,631 Germany Sept. 25, 1952 OTHER REFERENCES A.I.E.E. Transactions (publication), vol. 70, part 11, 1951, page 1814. 

5. A CABLE COMPRISING A CABLE CORE, A LONGITUDINALLY FOLDED METALLIC TAPE ENCLOSING SAID CORE WITH THE LONGITUDINAL EDGES OF THE METALLIC TAPE IN OVERLAPPING RELATION, A BRIDGING TAPE AXIALLY EXTENDING ALONG SAID METALLIC TAPE AND COVERING THE OVERLAPPING EDGES THEREOF WITHOUT BEING SECURED THERETO, AND A JACKET OF ELASTOMERIC MATERIAL EXTRUDED OVER SAID METALLIC TAPE AND SAID BRIDGING TAPE AND CONCENTRICALLY ENCLOSING SAID CABLE. 